Spring powered dispensers are in common use. For example in the medical field, there are automatically operated injectors for delivering medicaments to the tissues. Generally the device is placed on the patient's skin, and a release button is operated which unlatches a pre-loaded spring that drives the hypodermic needle through the epidermis, and thereafter pumps the medicament into the tissues. At the instant of release, the housing of the injector reacts against the mass of the driven piston in the reverse direction of the injection--that is, it recoils. This is wasted energy, although in the case of simple injectors, the recoil is resisted by the user's hand, and a larger proportion of the spring force is directed to moving the needle and medicament.
More sophisticated devices aim to apply a predetermined force to the skin, so that the optimum placement conditions are met before the device may be operated. Examples of the latter may be found in the needleless injectors disclosed in PCT/GB94/01608 (WO95/03844) by the present inventor. In these examples it is convenient to apply the force via a sleeve or ring acting through a spring, so that the user grasps the sleeve or ring, and presses the delivery orifice of the injector onto the skin. When the displacement of the sleeve reaches a predetermined value corresponding to the desired contact force on the skin, it operates a release mechanism which causes the injection. At the instant of release, the injector body, which is effectively "floating" within the operating sleeve, recoils away from the injection site, reacting against the operating sleeve spring. This represents wasted energy, since ideally, all of the spring energy should be directed to driving the medicament into the tissues, not in moving the injector body in the opposite direction.
It is possible to use a substantial spring to urge the injector via the operating sleeve onto the patient's skin, and thereby reduce the recoil by coupling the injector body through the spring and sleeve to the mass of the user's hand. However, this results in an unacceptably high pressure on the skin and/or trigger mechanism. For example, a needleless injector must be pressed onto the skin with a relatively light force for a subcutaneous injection, otherwise the subcutaneous tissues are compressed too much, resulting in a faulty injection.
Other devices which may employ pre-loaded spring energy to deliver a metered dose of liquid or powder, include breath-actuated metered dose inhalers (MDI's), automatic bactericide dispensers, and guns. In all cases where the user does not hold the device member on which the power spring reacts there is the potential of energy wastage. Many of these devices are intended for single use, or must be discrete--that is, small and lightweight, and it is important that the stored energy is used efficiently.